This invention relates to a magnetic resonance examining apparatus of heart beat synchronous type, and more particularly to an apparatus of the kind described above in which a nuclear magnetic resonance signal (which will be referred to hereinafter as an NMR signal) is obtained in synchronism with a heart beat of a subject to be examined, and a nuclear magnetic resonance image (which will be referred to hereinafter as an NMR image) is formed on the basis of the NMR signal.
An NMR image is formed from an NMR signal obtained in each of a plurality of times of spin excitation in a subject being examined. A clear NMR image cannot be obtained because the position where an NMR signal is generated in each time of spin excitation differs depending on parts of the heart moving due to a heart beat. Therefore, it has been a common practice to obtain an NMR signal by effecting spin excitation in synchronism with a heart beat.
In the art of imaging by scanning in synchronism with a heart beat, an electrocardiograph is usually used as a synchronous detector. FIG. 1 shows a general heart beat waveform recorded on the electrocardiograph. In FIG. 1, the period of the P and Q waves corresponds to a period 17 of contraction of the atrium, the period of the QRST waves corresponds to a period 18 of contraction of the ventricle, and the period between the end of the T wave and the beginning of the P wave corresponds to an expansion period 19.
The R wave has a highest peak, and this peak is usually used as a trigger for starting a signal read sequence of examination.
However, according to such a manner of scan imaging, spin excitation is effected only once for each heart beat which takes a period of time of about 0.8 to 1.3 sec, and the resultant data is read or fetched. Therefore, a method called a multiphase imaging method well known in the art from the disclosure of, for example, "Journal of NMR Medicine", Vol. 6, September, 1986, Page 119 is now employed. According to this method, the R wave of a heart beat waveform is used as a trigger, and, after a predetermined time from this R wave, spin excitation is repeatedly effected a plurality of times at intervals of a predetermined repetition time A from the delay time as shown in FIG. 2 so as to obtain a plurality of NMR signals. Thus, an NMR image, that is, a phase image is obtained at each phase of spin excitation.
In the method of multiphase imaging described above, spin excitation is effected at the interval of the repetition time A at each phase where the NMR image is obtained. However, no spin excitation is effected at the phase where the NMR image need not be obtained. Therefore, the value of a repetition time B, in which no NMR image is obtained, differs from that of the spin-excitation repetition time A, and this means that the spin excitation is not continuously effected at a constant period. As a result, the state of recovery of longitudinal relaxation after the spin excitation at each phase is not uniform, and the NMR signals generated from the subject at individual phases will differ from each other, resulting in different contrasts of the NMR images obtained at individual phases.